In the followings, details are given on NOx formation during combustion, on existing techniques for its reduction, and especially on its reduction in chemical recovery boilers at pulp mills.
Nitrogen Oxides from Combustion Processes (NOx=NO+NO2)
Nitrogen oxides are known pollutants whose emission is regulated for most combustion devices. During combustion NOx generate from a portion of the nitrogen carried by air and fuel, while the remaining nitrogen is emitted in form of molecular nitrogen (N2) and minor amounts of hazardous compounds such as nitrous oxide (N2O), ammonia (NH3), and hydrogen cyanide (HCN). The formation of NOx occurs via a number of routes, depending on conditions and fuels [1-7].
The purpose of all reducing techniques is to minimize the emission of polluting NOx, and thus to maximize the share of harmless N2 while maintaining low the emission of all other hazardous compounds.
Time is a most important variable for all NOx reducing techniques, and thus for the process of the present invention. Time requirements can be governed either by physics (i.e. the time needed for reactants to mix) or by chemistry (i.e. the time needed for the chemical kinetics). In this patent application, the term “time” will be used when referring to the time required in real devices (i.e. both the physics and chemistry time). However, the process of the invention is mainly controlled by chemical kinetics. Thus, the term “chemistry time” will be introduced when focusing solely on the limitations set by chemical kinetics.
Fuel Staging (FS)
Fuel staging has been utilized in combustion devices for power generation to reduce NOx emissions. Fuel staging consists of staging the combustion fuel in a number of streams, which are delivered at convenient locations [8] in a combustion device. This technique has been applied in a variety of combustion devices since the 1970s. The simplest FS features a sequence of fuel streams located along the furnace in a vertical direction so as to set a progressive increase of the stoichiometric ratio (SR) from extremely lean (SR>>1, mixtures containing a much higher amount of air than that indicated by the stoichiometric ratio) to the nominal excess air that warrant complete combustion (SR≧1). The reactions inset at each fuel stream provide the radicals that reduce previously formed NOx. Typical radicals are methyl-(CHi), ketenyl-(HCCO), and hydrogen (H).
A more effective design for FS is the so-called “reburning” [9]. In reburning a primary fuel is burned to completion with excess air (primary combustion, SRI>1), then reburn fuel is added to reset reducing conditions (reburn, SRII<1), and finally combustion is completed with burnout air (burnout, SRIII>1). The primary zone assures efficient energy conversion, but also creates conditions that undesirably lead to NOx. The reburn zone is intended to reduce such NOx by producing the precursors and radicals that drive the reduction of NOx. The burnout zone assures the (almost) complete oxidation of the combustibles while minimizing the re-formation of NOx.
The effective reduction of NOx by fuel staging is limited to narrow temperature windows, which are determined by operational conditions such as pressure, stoichiometry, presence of various hydrocarbons, presence of carbon monoxide, and presence of nitrogen compounds else than NO and N2. Two useful windows have been found by manipulating such variables: one is wide and located well above 1600 K (in practice above 1700 K) (high temperature reburning) [e.g., 10], while the other is narrow and located around 1500 K (low temperature reburning) [e.g., 11]. The first is used in furnaces for energy production, while the latter is used in waste incineration and in glass and steel industries. The two reburning techniques achieve NOx reductions of 50-70% and 45-55%, respectively. The above can be summarized as follows:

We note that in fuel staging other N-compounds can form. When it comes to HCN, all efforts are made either to minimize its formation by limiting the hydrocarbons in the fuel [12], or to allow time for its destruction by upsizing the furnace [11].
Air Staging (AS)
Air staging has been used since decades for reducing the formation of NOx by limiting the availability of promoters such as O2, O, and OH. Air staging consists of staging the combustion air in a number of streams, which are delivered at convenient locations in a combustion device. This technique leads to ˜50% NOx reduction. Similarly to FS, air staging is affected by parameters such as temperature, pressure, stoichiometry, availability of hydrocarbons, presence of N-compounds other than NO and N2, but also by the number of stages [13]. Most favourable options include 3-5 air stages, whose stoichiometry is irrelevant as long as it increases while staying below 1 (SR<1) till the last stage [14]. This can be summarized as follows:

Air staging was first applied in conventional furnaces, but with time has been adjusted and renamed for other applications, e.g. “late air-staging” for fluidized bed combustors [15] and “rich-lean combustion” for gas turbines [16]. In the same way as in FS, also in air staging, efforts a re taken to limit HCN, whose formation is undesirably enforced by the reducing conditions (SR<1) [16].
Selective Non-Catalytic Reduction (SNCR)
Selective non-catalytic reduction consists of driving the reduction of combustion-generated NOx via the addition of an agent [17]. This technique leads to approximately 50% NOx reduction, and its efficiency depends on the operational conditions, the fuel composition, and the agent. Accordingly, many variants of this technique have been patented, including a fuel-lean process with ammonia [18], a fuel-rich process with ammonia [19], and a fuel-rich process with urea [20]. The fuel-rich SNCR process with ammonia (NH3) can be summarized as follows:

Variants of the SNCR include the addition of reducing agent via different streams, e.g., with the reburn fuel, with the air, or alone [11]. Each variant is limited to work under well-defined conditions. In absence of carbon monoxide (CO), fuel-lean SNCR works in the ranges 1100-1400 K, while fuel-rich SNCR works at higher temperatures. However, CO is present in nearly all processes where SNCR is used, with the detrimental consequence of shifting and narrowing the temperature windows. The conditions for optimal SNCR are hard to meet in many combustion devices.
Sequential Techniques
The aforementioned techniques (FS, AS, and SNCR) are often applied separately, but are also applied in sequential couples (“Hybrid reburn”) or triplets (“Advanced reburn”). Hybrid reburn (HR) enlists reburning and SNCR, leading to 80-90% reduction [21]. Advanced reburn (AR) consists of a sequence of reburning, SNCR, and air staging, and leads to an outstanding NOx reduction of over 90% [22] (FIG. 1). Several configurations have been proposed for these two sequential techniques. In most cases, each component has been optimised individually, thus achieving a cumulative effect on NOx, but also extending the limits of each component to the overall system [e.g. 22]. To the best of our knowledge, only Chen et al. [21] and Folsom et al. [23] have attempted to optimise all components in synergy. Nevertheless, such optimisations have led to requirements that hardly can be applied in furnaces. For instance, precisely controlled stoichiometry ratios (0.99<SRII<1 and 1<SRIII<1.02) have been found necessary at reburn and burnout. More in general, we remark that all sequential techniques have been developed considering HCN detrimental, and thus forcing its concentration as low as possible along the process [e.g., 24].
NOx in Chemical Recovery Boilers
Black liquor received from chemical pulp production is usually burnt in a recovery boiler. As the organic and carbonaceous substances are burning, the inorganic compounds in the black liquor turn into chemicals that are recovered and reutilized in the pulping process. The organic part of the black liquor is turned into energy. The black liquor is introduced through liquor spraying devices in the form of small drops into a furnace of the boiler. To ensure complete combustion, combustion air is also introduced into the recovery boiler. Air is usually introduced at three different levels: primary air at a lower part of the furnace, secondary air above the primary air level but below the liquor nozzles, and tertiary air above the liquor nozzles. These three air levels are conventional basic air levels in a modern recovery boiler, but other or additional air levels may be provided in the recovery boiler. The combustion of black liquor produces also nitrogen oxides.
The reduction of NOx in recovery boilers has been addressed already by techniques based on staging or SNCR via i) the “quaternary air” in the upper portion of the boiler (upper furnace) [26], ii) the “vertical air staging” [27,29] in which air jets are fed into the furnace of the recovery boiler from nozzles located on at different elevations and in a pattern of vertical space-apart rows, iii) the “Mitsubishi Advanced Combustion Technology” (MACT) [28] where a reducing agent (urea) can be added after staging, and iv) black liquor staging [30] in which black liquor is fed from at least two levels into a furnace equipped with vertical air staging (ii). These techniques have proved 30-50% NOx reduction, but require operational adjustments that deviate from the optimum for the boiler. Also, these techniques require upsized boilers in order to maintain the temperature in the furnace sufficiently low, thus minimizing the conversion of fuel-N to ammonia (NH3) and finally NOx. In practice, staged combustion in recovery boilers demands temperatures as low as 850-1000° C., which can be achieved only in boilers that are larger and thus more expensive than the conventional ones.
In addition to the techniques listed above (i-iv) we refer here to two more techniques that have been disclosed in two patent publications.
First, the patent application JP 7112116 discloses a method of reducing the amount of NOx in the exhausts of a black liquor recovery boiler by introducing additional black liquor in the upper furnace, above the conventional introduction level. In this stage ammonia and hydrogen cyanide are formed from the pyrolysis of the additional black liquor. It can be stated that this stage is arranged to generate reactants, i.e. NH3 and HCN, for SNCR (Selective non-catalytic reduction of NOx). In this system the targeted reaction is the direct reduction of NOx (formed in the primary combustion stage) under oxidizing conditions in the upper part of the furnace where the flue gas temperature is suitable, typically 850-1050° C. For instance, the direct reduction of NO by NH3 is resumed by the overall reaction:NH3+NO+0.25 O2→N2+1.5 H2Owhile the reduction of NO by HCN is resumed as:NO+HCN+0.75 O2→N2+0.5 H2O+CO2 In these reactions oxygen is needed, as in all SNCR-systems, i.e. oxidizing conditions are needed in the reaction zone, where the additional black liquor is introduced.
Secondly, the patent FI 102397 discloses a process in which additional fuel is introduced into the recovery boiler above the usual liquor level. Here, the combustion conditions remain substoichiometric and the temperature is as unfavourable as possible for the formation of NOx. The additional fuels originate from the pulp manufacturing process and may be, for example, a malodorous gas or soap. These fuels contain hydrocarbons which, when burning, generate radicals that intensify the reactions of nitrogen compounds and finally reduce the amount of NOx compounds. After supply of the additional fuel, excess air is fed to the boiler for final combustion.
An object of the present invention is to provide a method of controlling emissions of harmful nitrogen compounds, and especially nitrogen oxides, from combustion processes in a more efficient and economical way as compared to the techniques described above.